Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B44—DECORATIVE ARTS
  • B44F—SPECIAL DESIGNS OR PICTURES
  • B44F1/00—Designs or pictures characterised by special or unusual light effects
  • B44F1/08—Designs or pictures characterised by special or unusual light effects characterised by colour effects
  • B44F1/10—Changing, amusing, or secret pictures
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
  • B41M3/00—Printing processes to produce particular kinds of printed work, e.g. patterns
  • B41M3/14—Security printing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/20—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof characterised by a particular use or purpose
  • B42D25/29—Securities; Bank notes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/30—Identification or security features, e.g. for preventing forgery
  • B42D25/328—Diffraction gratings; Holograms
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/30—Identification or security features, e.g. for preventing forgery
  • B42D25/355—Security threads
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/30—Identification or security features, e.g. for preventing forgery
  • B42D25/36—Identification or security features, e.g. for preventing forgery comprising special materials
  • B42D25/364—Liquid crystals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/30—Identification or security features, e.g. for preventing forgery
  • B42D25/36—Identification or security features, e.g. for preventing forgery comprising special materials
  • B42D25/373—Metallic materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
  • B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
  • B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
  • B42D25/30—Identification or security features, e.g. for preventing forgery
  • B42D25/36—Identification or security features, e.g. for preventing forgery comprising special materials
  • B42D25/378—Special inks
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/285—Interference filters comprising deposited thin solid films
  • G02B5/286—Interference filters comprising deposited thin solid films having four or fewer layers, e.g. for achieving a colour effect
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T156/00—Adhesive bonding and miscellaneous chemical manufacture
  • Y10T156/10—Methods of surface bonding and/or assembly therefor

Definitions

• the present invention is in the field of security documents. It concerns a paired optically variable security element which comprises first and second optically variable interference devices in the form of optically variable foils or of printings made with ink comprising optically variable interference pigments, wherein said first and second interference devices exhibit spectral matching at a determined angle of incidence.
• sets of optically variable devices and of inks or coating compositions comprising optically variable pigments for the production of said paired optically variable security element are also disclosed, as well as the use of said security element for the protection of documents and goods, and security documents and goods carrying said security element.
• Optically variable foils optically variable pigments (OVP), and coating compositions comprising OVP, noteworthy optically variable ink (OVI®), are known in the field of security printing.
• OVP optically variable pigments
• OPI® noteworthy optically variable ink
• Such optically variable elements exhibit a viewing- angle or incidence-angle dependent color, and are a preferred choice to protect banknotes and other security documents against the illegal reproduction by commonly available color scanning, printing, and copying office equipment.
• WO 2005/044583 discloses the use of a same optically variable security element in more than one constituting parts of a security document.
• WO 96/39307 discloses a paired optically variable device, comprising first and second optically variable devices in spaced apart positions on a same surface, having first and second optically variable pigments disposed in said first and second optically variable devices, respectively, wherein said optically variable pigments have the same color at one determined angle of incidence, and different colors at all other angles of incidence.
• the device of WO 96/39307 is characterized in that the designs for the paired optically variable pigments are selected such that, in the (CIELAB) a*b* diagrams, representing the color of the said pigments as a function of the viewing or incidence angle, there are crossover points corresponding to viewing or incidence angles at which two of said optically variable pigments have the same hue.
• the first and the second optically variable pigments of WO 96/39307 are embodied, in the case of all-dielectric interference pigments, as different quarter-wave designs at an approximately same design wavelength. In the case of metal-dielectric interference pigments, the first and the second pigments are embodied as different half- wave designs at an approximately same design wavelength.
• the principal shortcoming of the device of WO 96/39307 is that the required "same hue" of the first and the second pigment at one determined angle of incidence must be produced through different spectral characteristics, because it is impossible to realize the same spectral characteristics using different quarter-wave or half-wave designs.
• the observed hue merely represents the projection of the pigment's spectral reflection characteristics, i.e. the reflection intensity as a function of wavelength, onto the 3-dimensional space of human color perception, and, as known to the skilled person, different spectral characteristics can have the same projection onto the space of human color perception ⁇ color metamerism).
• a paired optically variable security element having first and second optically variable interference devices, embodied e.g. as first and second optically variable foils, or first and second optically variable pigments in an ink or coating composition, which devices are arranged such that they can be viewed together, and which exhibit a true spectral matching at a determined angle of incidence, whilst having different spectra at all other angles of incidence.
• first and second optically variable interference devices embodied e.g. as first and second optically variable foils, or first and second optically variable pigments in an ink or coating composition
• the paired optically variable security element of the present invention comprises thus at least a first and a second optically variable interference device, said interference devices having different color travels, and being embodied either as all- dielectric multilayer stacks, or as metal-dielectric multilayer stacks, or as choles- teric (i.e. chiral nematic) liquid crystal films, or as combinations thereof.
• Said interference devices are further characterized in that they have a same, e.g. a quarter-wave or a half- wave, interference design at the angle of incidence of spectral matching ⁇ crossover point), and in that they differ in the refractive index of at least one of their constituting dielectric layers.
• the present invention relies on the fact that the color travel of an interference device depends on the refractive index of the dielectric materials comprised in the device. This dependency is a physical law, related to the difference in light propa- gation speed inside and outside the different layers of the interference device, and therefore applies to all type of color- shifting optical interference devices, be they of the all-dielectric multilayer thin-film, of the metal-dielectric multilayer thin- film, or of the cholesteric liquid crystal type.
• Color travel and “color-shift”, within the present disclosure, mean the color change which is observed when turning the optically variable interference device from orthogonal to grazing incidence.
• Color travel more precisely refers to the color of the said device as a function of the viewing or incidence angle the in the (CIELAB) a*b* diagram, whereas “color-shift” only refers to the changing visual appearance of the device.
• Optically variable in the context of the present disclosure, refers to the property of viewing- or incidence-angle dependency of color.
• “Orthogonal incidence” means viewing at an angle of 80° to 90° with respect to the plane of the interference device.
• Gram incidence means viewing at an angle of 0° to 10° with respect to the plane of the interference device. Illuminating under specular conditions is generally assumed.
• a “crossover point” is a viewing or incidence angle at which said first and second optically variable devices have the same hue in the (CIELAB) a*b* diagram.
• “Spectral matching”, within the present disclosure, means that the spectral reflec- tion or transmission characteristics, as a function of the wavelength, are qualitatively similar; i.e. that the spectral characteristics of the first and the second optically variable interference device exhibit the same spectral bands, at the same wavelengths, and having the same broadness. "Spectral matching”, in the following, does not mean that the spectral bands of the first and the second optically variable device have the same absolute intensities. In fact, different absolute reflection or transmission intensities of the first and the second device, such as can have their origin in the use of different materials, or in differing pigment loadings, are admissible in the context of present invention.
• the origin of the reflection color, as well as of the observed color shift with angle, of the herein contemplated interference devices is in the following explained at the example of a half-wave-design metal-dielectric thin-film interference device. Similar reasoning applies, with the necessary change, for the transmission color of interference devices used in translucency, as well as for the other types of inter- ference devices, namely the quarter-wave designs, the all-dielectric thin-film devices, the cholesteric liquid crystal films, and the possible combinations of such devices and designs.
• a half-wave-design metal-dielectric thin-film interference device is characterized in that it comprises an "absorber / dielectric / reflector" layer structure, wherein the "absorber” layer is partially transmitting and partially reflecting, the dielectric layer is transmitting, and the reflecting layer is reflecting the incoming light.
• An illustrative embodiment of such device is given by a layer sequence and thickness of: "Chromium (5nm) / Magnesium fluoride (400nm) / Aluminum (40nm)". Referring to Fig.
• optical lag OL
• n*d*sin( ⁇ ') the apparent optical thickness of a dielectric layer (D) of refractive index n > 1
• ⁇ ' denotes the incidence angle of the light, with respect to the plane of the layer, Jn 1 side the layer (D).
• a light wave traveling forth through the dielectric (D), being reflected at the reflector layer (R), and traveling back through the dielectric (D), lags thus behind a light wave reflected at the topmost absorber layer (A) by an amount of 2*OL 2*n*d*sin( ⁇ '), as seen from inside the layer (D).
• sin( ⁇ ') can be expressed as a function of the incidence angle ⁇ , with respect to the plane of the layer, outside the layer.
• OL d*V(n 2 - cos 2 ( ⁇ )).
• a light wave traveling forth through the dielectric (D), being reflected at the reflector layer (R), and traveling back through the dielectric (D), lags thus behind a light wave reflected at the topmost absorber layer (A) by an amount of 2*OL 2*d*V(n 2 - cos 2 ( ⁇ )), as seen from outside the layer (D).
• the root sign V denotes herein the square root of the following argument in parentheses.
• the paired optically variable security element is based on first and second optically variable pigments originating from a same interference design, e.g. a same quarter-wave or a same half-wave design, at the angle of incidence of spectral matching ⁇ crossover point).
• a same interference design e.g. a same quarter-wave or a same half-wave design
• the refractive indices of at least one of the constituting dielectric layers of the first and second optically variable pigments must be chosen different, thus resulting in a different color shift with incidence angle, whilst having the same spectral reflection or transmission charac- teristics, i.e. truly a same color, at the crossover point.
• the color matching in the present case is a true, exact match of spectral reflection or transmission characteristics at a determined angle of incidence, and the authentication of a document or item through the matching colors of both parts of the paired optically variable security element disclosed herein at a determined angle of incidence does not depend on the chosen illumination conditions.
• the paired optically variable security element of the present invention comprises thus a first optically variable interference device having a first dielectric of a lower refractive index (n low ), and whose k th -order reflection maximum (k) shows a shift from a first reflection wavelength ( ⁇ i) at orthogonal incidence to a second, shorter wavelength ( ⁇ 2 ) at grazing incidence, and a second optically variable interference device having a second dielectric of a higher refractive index (nh lg h), and whose same k th -order reflection maximum (k) shows a shift from a third reflection wavelength ( ⁇ 3 ) at orthogonal incidence to a fourth, shorter wavelength ( ⁇ 4 ) at grazing incidence, the security element being characterized in that in that the range spanned up by said third and said fourth wavelengths of said second device is within the range spanned up by said first and said second wavelengths of said first device.
• This latter aspect is a necessary condition for the existence of an an- gle of incidence, under which
• the first and second optically variable interference devices must be of the same half-wave or quarter-wave design, in order to produce a spectral matching, and can be embodied by a design chosen from the group consisting of the all-dielectric multilayer stacks, the metal-dielectric multilayer stacks, the cholesteric liquid crystal films, and the combinations thereof.
• the said interference devices are embodied by optically variable foils.
• the optically variable foils may comprise an "absorber / dielectric / reflector" layer sequence, wherein the reflector layer may be followed by further layers, and means to affix the foil to a substrate such that the "absorber" layer is on the outside.
• the said interference devices are embodied by optically variable pigments comprised in corresponding inks or coating compositions and applied to a document or good to be protected.
• the optically variable pigments may comprise an "absorber / dielectric / reflector / dielectric / absorber" layer sequence, wherein the reflector layer may comprise further, internal layers.
• the optically variable pigment is incorporated into plastic foils, so as to result in another type of optically variable foils.
• the optically variable pigment can herein either be incorporated into the plastic mass used to cast the foil and oriented through a controlled stretching of the foil (e.g. through calendering).
• the optically variable pigment can be laminated between two plastic foils, to make up a single optically variable foil.
• a combination of inks or coatings comprising optically variable pigments, foils containing optically variable pigment, and optically variable foils can also be used to embody the security element of the present invention, foreseen that the required spectral matching conditions are fulfilled.
• the security element can furthermore be embodied in or on a transparent or a translucent substrate, for viewing in translucency, or on an opaque substrate, for viewing in reflection.
• the security element of the present invention can be embodied in the forms of optically variable printings made with ink on a substrate, of opti- cally variable foils affixed to a substrate, of security threads incorporated into a substrate, or of transparent window substrates.
• a process for making a paired optically variable security element comprising the steps of - applying to a substrate (S) a first optically variable interference device having a first dielectric of a lower refractive index (ni ow ), and whose k th -order reflection maximum (k) shows a shift from a first reflection wavelength ( ⁇ i) at orthogonal incidence to a second, shorter wavelength ( ⁇ 2 ) at grazing incidence;
• a second optically variable interference device having a second dielectric of a higher refractive index (nh lg h), and whose same k th - order reflection maximum (k) shows a shift from a third reflection wavelength ( ⁇ 3 ) at orthogonal incidence to a fourth, shorter wavelength ( ⁇ 4 ) at grazing incidence; wherein said first and said second optically variable device are selected such that the range spanned up by said third and said fourth wavelengths of said second device is within the range spanned up by said first and said second wavelengths of said first device.
• the first and the second optically variable interference devices are of the same half-wave or quarter-wave design and preferably arranged such that they can be viewed together.
• the paired optically variable security element according to the present invention can be used for the counterfeit-protection of a document, such as a banknote, a value document, an identity document, an access document, a label, or a tax excise stamp, as well as for the marking of a good.
• a document such as a banknote, a value document, an identity document, an access document, a label, or a tax excise stamp
• a security document such as a banknote, a value document, an identity document, an access document, or a tax excise stamp, which comprises a paired optically variable security element according to present invention.
• the present invention comprises also a set of first and second optically variable interference devices for embodying a paired optically variable security element, said first optically variable interference device having a first dielectric of a lower refractive index (n low ), and whose k th -order reflection maximum (k) shows a shift from a first reflection wavelength ( ⁇ i) at orthogonal incidence to a second, shorter wavelength ( ⁇ 2 ) at grazing incidence, and said second optically variable interfer- ence device having a second dielectric of a higher refractive index (nh lg h), and whose same k th -order reflection maximum (k) shows a shift from a third reflection wavelength ( ⁇ 3 ) at orthogonal incidence to a fourth, shorter wavelength ( ⁇ 4 ) at grazing incidence, wherein the range spanned up by said third and said fourth wavelengths of said second device is within the range spanned up by said first and said second wavelengths of said first device.
• said first and said second optically variable interference device can be chosen from the group consisting of the optically variable foils, the optically variable threads, and the optically variable windows.
• the invention comprises further a set of first and second optically variable coating compositions, in particular inks, for embodying a paired optically variable security element, said first coating composition containing a first optically variable interference pigment having a first dielectric of a lower refractive index (n low ), and whose k th -order reflection maximum (k) shows a shift from a first reflection wavelength ( ⁇ i) at orthogonal incidence to a second, shorter wavelength ( ⁇ 2 ) at grazing incidence, and said second coating composition containing a second optically variable interference pigment having a second dielectric of a higher refractive index (nh lg h), and whose same k th -order reflection maximum (k) shows a shift from a third reflection wavelength ( ⁇ 3 ) at orthogonal incidence to a fourth, shorter wavelength ( ⁇ 4 ) at grazing incidence, wherein the range spanned up by said third and said fourth wavelengths of said second pigment is within the range spanned up by said first and said second wavelengths of said
• said first and said second optically variable coating composition can be chosen from the group consisting of the screen printing inks, the copperplate- intaglio inks and the gravure printing inks.
• Fig. 1 illustrates the origin of the perceived color and color- shift of an absorber / dielectric / reflector / thin- film multilayer stack, such as can be used in the present invention.
• Fig. 2 illustrates the physical working principle of the security element according to the present invention: a) orthogonal incidence spectrum of a CrZMgF 2 / Al metal- dielectric interference stack; b) orthogonal incidence spectrum of a CrAf 2 OsZAl metal-dielectric interference stack; c) grazing incidence spectrum of a CrZY 2 OsZAl metal-dielectric interference stack; d) grazing incidence spectrum of a CrZMgF 2 ZAl metal-dielectric interference stack.
• Fig. 3 schematically illustrates the paired optically variable security element of the present invention, embodied by indicia of a first (right part of the image) and of a second (left part of the image) interference device: a) the paired security element seen under orthogonal incidence
• Fig. 1 illustrates the origin of the perceived color and color- shift of an absorber / dielectric / reflector / thin- film multilayer stack, such as can be used in the present invention: a reflector layer (R), which may have an internal layer structure, carries at least one dielectric layer (D), which, in turn, carries at its outer surface an ab- sorber layer (A).
• R reflector layer
• D dielectric layer
• A ab- sorber layer
• the primary reflected beam (Ii) and the secondary reflected beam (I3) interfere with each other, causing certain wavelengths to be partially or totally extinguished and others not (destructive and constructive interference), hence producing the appearance of color through the selective reflection of particular parts of the white-light spectrum.
• Optically variable thin-film interference devices having an all-dielectric or a metal-dielectric design can be produced, as known to the skilled person, by the successive physical- vapor-deposition (PVD) of the different materials constituting the thin-film device onto a suitable carrier substrate, such as described in US 4,705,356; US 4,838,648; US 4,930,866; US 5,084,351; US 5,214,530; US 5,278,590; EP-B-O 227 423; as well as in EP-B-I 366 380 and the hereto related documents.
• the carrier is preferably a flexible web, e.g. a release-coated polyethylene tere- phthalate (PET) foil.
• the vapor-deposition can be carried out as a roll-to-roll process in a high vacuum coater.
• the materials are evaporated using material- specific, appropriate evaporation sources and processes known to the skilled person, such as sputtering, reactive sputtering, magnetron sputtering, thermal evaporation, electron-beam or laser-beam assisted evaporation.
• CVD chemical vapor deposition
• wet coating in particular sol-gel coating processes.
• PVD physical vapor deposition
• CVD chemical vapor deposition
• a precursor material e.g. Ti
• a present gas phase e.g. O 2
• depositing as a reaction product e.g. TiO 2
• Cholesteric liquid crystal films are known from WO9409086A1, EP0601483Al, US5502206, EP0661287B1, EP0686674B1, US5683622, EP0709445B1, EP0712013A2, WO9729399A1, EP0875525A1, EP0885945A1, as well as from related documents known to the skilled person.
• Such foils are ob- tained by coating a carrier foil with a polymerizable cholesteric liquid crystal precursor mixture, followed by orienting the liquid crystal in the cholesteric phase at an appropriate temperature and fixing it by polymerization, e.g. by UV-curing.
• Corresponding cholesteric liquid crystal polymer (CLCP) pigments are obtained from such foil by comminuting it to the desired particle size. Coating composi- tions containing such pigments are disclosed in US5807497, EP0758362A1, WO9532247A1, EP0887398A1, as well as in related documents known to the skilled in the art.
• the refractive index of the cholesteric liquid crystal polymer can be varied through an appropriate choice of the chemistry used.
• a high number of cross- linkable monomers and oligomers are noteworthy known to form cholesteric phases under appropriate conditions, which phases can be "frozen” in a determined state through radiation-induced or otherwise induced cross-linking reac- tions.
• Monomers and oligomers which are free of aromatic residues like benzene, naphthalene and other conjugated cycles result in low-refractive-index cholesteric liquid crystal polymers. Examples of this type are the liquid crystal polymers derived from cholesterol.
• a previously embossed and release-coated carrier foil e.g. of PET
• the embossing is carried out with the help of a heated embossing shim, as known to the person skilled in the manufacturing of surface holograms.
• the embossed relief pattern in the carrier foil is subsequently reproduced by the optically variable multilayer interference device which is vapor- deposited on top of this carrier foil, or by the liquid crystal film which is produced on top of this carrier foil.
• the carrier foil coated with an optically variable interference device can also be converted, according to known procedures, to a hot- or cold-stamping transfer foil for the copy-protection of documents.
• the optically variable interference device film is detached from the carrier foil and comminuted into pigment, so as to result in pigment flakes having a particle size ranging from 200nm to 3'000nm thickness, preferably ranging from 400nm to 5000nm thickness and a particle diameter ranging from 5 to 50 micrometers extension.
• Said comminution can be advantageously performed with the help of a jet mill, and the resulting particles are preferably classified into appropriate size fractions.
• the resulting optically variable pigment is preferably formulated into a printing ink, which may comprise it in amounts ranging from 1% to 25% by weight, together with at least one organic polymer or polymer precursor as a binder, and, where appropriate, other types of pigments, in particular coated particles and/or iridescent pigments, conventional dyes, inorganic and organic printing pigments such as described in O. L ⁇ ckert, Pigment + F ⁇ llstoff Kunststoff Kunststoff, 5 th edition, Laatzen, 1994, as well as extenders, rheology additives, solvents, photosensitizers and sicActivting agents.
• Other security materials may also be present in the ink, such as magnetic pigments, luminescent pigments or dyes, and infrared-absorbing pigments or dyes, etc.
• the ink composition is preferably formulated for the screen-printing process, such as to have a viscosity in the range of 0.5 to 2 Pa.s at 40 0 C; however, other preferred options include inks for the copperplate-intaglio printing process, having a viscosity in the range of 2 to 20 Pa.s at 40 0 C and inks for the flexo gravure printing process, having a viscosity in the range of 0.1 to 0.5 Pa.s at 40 0 C.
• the formu- lation of such inks is known to the skilled person.
• the resulting optically variable inks can be used for the printing of indicia on an item to be protected, e.g. a security document, said indicia being embodied as a paired optically variable device such that they can be viewed together.
• the so obtained, optically variable security feature is easily detected by the human eye, e.g. by comparing both optically variable devices forming the paired optically variable feature and checking for a determined incidence angle where they have the same spectral reflection or transmission characteristics.
• Such comparison which is truly independent of the environment lighting conditions, allows, by simple visual examination, to judge the authenticity of a document carrying the paired optically variable security feature of the present invention.
• magnetic optically variable pigment e.g. according to US 4,838,648 or EP-B-I 366 380
• the magnetic pigment flakes in the ink are further oriented (e.g. according to EP-B-I 641 624), during or after the printing process, by applying a corresponding magnetic field, and the position of the so oriented flakes is subsequently fixed by hardening the ink.
• UV-curable ink formulations are used for this ap- plication; such formulations can be prepared as known to the skilled person.
• First and second optically variable interference devices were prepared by the successive physical vapor deposition of the different layers of each a symmetric half- wave metal-dielectric interference design onto a release-coated PET carrier foil.
• Absorber layer Cr, 3.5 nanometers
• Reflector layer Al, 40 nanometers
• Absorber layer Cr, 3.5 nanometers
• Reflector layer Al, 40 nanometers
• Absorber layer Cr, 3.5 nanometers
• the CrZMgF 2 / Al metal-dielectric interference stack of the first device shows a reflection spectrum as indicated in Fig. 2a, having the 3 rd -order reflection maximum in the blue-green, at 500 nm.
• the 2 nd - order reflection maximum (k) is in the red, at 660 nm (first wavelength, ⁇ i).
• spectrum in Fig. 2d the 2 nd -order reflection maximum moves up into the blue, at 445 nm (second wavelength, X 2 ).
• the CrAf 2 OsZAl metal-dielectric interference stack of the second device shows a reflection spectrum as indicated in Fig. 2b, having the 2 nd -order reflection maximum in the orange, at 600 nm (third wavelength, ⁇ ? ).
• spectrum in Fig. 2c the 2 nd -order reflection maximum moves up into the blue-green, at 510 nm (fourth wavelength, ⁇ 4 ).
• the range spanned up by said third and said fourth wavelengths of said second device is thus within the range spanned up by said first and said second wavelengths of said first device.
• the crossover point is situated at a viewing or incidence angle ⁇ of 40 0 C, where both optical interference paths through the dielectric layer of the first and the second interference devices are equal.
• the 2 nd -order reflection maximum is at 545 nm, and both devices show the same, grass-green interference color (Fig. 3b).
• the so obtained interference devices can be converted to stamping foils, together forming a set of first and second optically variable interference devices for embodying a paired optically variable security element according to the present in- vention.
• the obtained interference devices can be removed from the carrier foils, comminuted into pigments and converted into printing inks according to methods known to the skilled and described in the art, together forming a set of first and second optically variable coating compositions for embodying a paired optically variable security element according to the present invention.
• Exemplary printing ink formulations can be prepared as follows:
• the paired optically variable security element of the present invention can be printed in the form of indicia on a security document, such as a banknote, a value document, an identity document, an access document, a label, or a tax excise stamp, or on a commercial good.
• a security document such as a banknote, a value document, an identity document, an access document, a label, or a tax excise stamp

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• 2009
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